* 2010 drought released more emissions than 2005 dry spell
* Frequent droughts may turn forest into emissions source
* Droughts fit some models of drier Amazon this century
By Stuart Grudgings
RIO DE JANEIRO, Feb 3 (Reuters) - A widespread drought in the Amazon rain forest last year was worse than the "once-in-a-century" dry spell in 2005 and may have a bigger impact on global warming than the United States does in a year, British and Brazilian scientists said on Thursday.
More frequent severe droughts like those in 2005 and 2010 risk turning the world's largest rain forest from a sponge that absorbs carbon emissions into a source of the gases, accelerating global warming, the report found.
Trees and other vegetation in the world's forests soak up heat-trapping carbon dioxide as they grow, helping cool the planet, but release it when they die and rot.
"If events like this happen more often, the Amazon rain forest would reach a point where it shifts from being a valuable carbon sink slowing climate change to a major source of greenhouse gases that could speed it up," said lead author Simon Lewis, an ecologist at the University of Leeds.
The study, published in the journal Science, found that last year's drought caused rainfall shortages over a 1.16 million square-mile (3 million square km) expanse of the forest, compared with 734,000 square miles (1.9 million square km) in the 2005 drought.
It was also more intense, causing higher tree mortality and having three major epicenters, whereas the 2005 drought was mainly focused in the southwestern Amazon.
As a result, the study predicted the Amazon forest would not absorb its usual 1.5 billion tonnes of carbon dioxide from the atmosphere in both 2010 and 2011. In addition, the dead and dying trees would release 5 billion tonnes of the gas in the coming years, making a total impact of about 8 billion tonnes, according to the study.
In comparison, the United States emitted 5.4 billion tonnes of carbon dioxide from fossil fuel use in 2009.
The combined emissions caused by the two droughts were probably enough to have canceled out the carbon absorbed by the forest over the past 10 years, the study found.
The widespread drought last year dried up major rivers in the Amazon and isolated thousands of people who depend on boat transportation, shocking climate scientists who had billed the 2005 drought as a once-in-a-century event.
The two intense dry spells fit predictions by some climate models that the forest will face greater weather extremes this century, with more intense droughts making it more vulnerable to fires, which in turn could damage its ability to recover.
Under the more extreme scenarios, large parts of the forest could turn into a savannah-like ecosystem by the middle of the century with much lower levels of animal and plant biodiversity. Although human-caused deforestation in Brazil has fallen sharply in recent years, scientists say the forest is still vulnerable.
A crucial question is whether the droughts are being driven by higher levels of greenhouse gases or are an anomaly, Lewis said. If they are driven by global warming, a vicious cycle of warmer temperatures and droughts could conceivably lead to a large-scale transformation of the forest over a period of decades.
"You could quite rapidly move to a much drier Amazon with less forest there," Lewis told Reuters.
The research was a collaboration among scientists at the University of Leeds and the University of Sheffield in Britain and Brazil's Amazon Environmental Research Institute.
Editing by Will Dunham